The PHILCO Phorum
PT-6 finished!! - Printable Version

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RE: PT-6 finished!! - bridkarl - 08-05-2025

Ok. Pin 3 on the 7c6 is the grid. Thanks. I’ll mark this in my notes.

RE: PT-6 finished!! - bridkarl - 08-07-2025

Being dumb today - my wife says it’s every day the older I get - define “bottom” - the end on the tube socket or the other end?

RE: PT-6 finished!! - Vlad95 - 08-07-2025

my wife says it’s every day the older

We don't age every day, we grow up.

RE: PT-6 finished!! - MrFixr55 - 08-09-2025

Find the 0.1uF cap #6 on the diagram. Measire across this cap. This is your AVC voltage. place the + lead on the end of the cap that goes to B- and the - lead on the end of the cap that goes to the antenna and IF secondaries. That end is the AVC line.

Hope this helps.

RE: PT-6 finished!! - bridkarl - 08-09-2025

Thanks. I expect that is generally the same across many Philcos of the 1941-42 period which is what I’m interested in so this is very useful.

RE: PT-6 finished!! - MrFixr55 - 08-10-2025

The basic circuit of the Transitone is very similar all the way into the 1950s. There are 2 basic AVC circuits used in inexpensive AA5 radios:
In one circuit, a separate diode from the detector is used. This is coupled to IF via a 100 pF cap and a resistor. The output of this diode passes through a 2.2 Megohm resistor and filtered by a 0.1uF capacitor to provide a negative DC AVC voltage. One advantage of this circuit is that there is better response from the detector itself with somewhat less distortion. It seems that every Philco that uses a 7A8 or 14A8 uses this circuit, and was used from 1941 - 1952 5 tube AC-DC models.

The other main style of AVC circuit merely takes the AVC signal off the detector circuit at the volume control through a 2.2 megohm resistor. This is literally pulsating DC. The 0.1 uF cap acts as a filter. This is used on most Philcos post 1953.

Both of these circuits provide a negative DC voltage at the capacitor that is used to bias the IF and antenna circuits. The stronger the AVC signal, the more negative the bias, therefore the weaker the gain. The less signal strength, the less negative bias, therefore the stronger the gain.

RE: PT-6 finished!! - bridkarl - 08-10-2025

I have this all finished and back in the case. Seems to work fine. My only remaining parts issue is I’m missing the dial scale strap and the associated screw. Plus it could use a new back. And probably an alignment, dial string, and some case work.

I’ll keep watching online for that. We only have one AM station here so it’s fine for now.
Just a repair bench radio on my spare isolation transformer.